This invention relates to solar panel having an improved design, whereby a much increased volume of solar radiation can be collected compared with known solar panels.
It is well known that any surface preferably blackened, when exposed to solar radiation, whether direct or diffuse, increases in temperature. If the surface has some means whereby the radiation reaching it can be transferred to some other medium, such as water or air, then the device is acting as a `solar heater`. Methods of improving the performance of such heaters include (1) the provision of one or more glass or translucent plastic covers on the front side, establishing the well known "Greenhouse Effect", (ii) the addition of appropriate insulation at the back of the collector surface and at the sides, (iii) reducing the emissivity of the surface in the long wave lengths, while keeping its emissivity high for incoming solar radiation, usually by means of a "selective surface". Solar heaters can be classified into two main groups, one being "concentrators", in which the direct sunbeam is focussed to some central point (often a cylindrical pipe is placed in a parabolic trough), the other being "flat plate collectors", and a wide variation of materials have been used.
The conventional flat plate collector is well established for both domestic water heating and for space heating applications. Its use in the United Kingdom has been described for example in a recent report by the United Kingdom section of the International Solar Energy and by the recently published United Kingdom government report. Among its advantages are (a) the ability to raise the water temperature to a level where it can make a significant contribution to the domestic and/or space heating demand (b) the ability to achieve this, during the summer months, at reasonable efficiencies, typically in the order of 50% or more during periods of good radiation and (c) the ability to supply heat without the necessity of any tracking device or mechanism. The major drawback to the use of the simple flat-plate collector in the United Kingdom is the relative weakness of the solar radiation level especially during the winter months. Another feature which tends to act against more widespread use of the simple black flat-plate collector is its appearance. Areas of black surface on a light coloured roof or wall can be considered to be detrimental to the overall visual senerity.
The compound parabolic concentration (CPC) was first described by Winston & Hinterberger in 1966 and serious work on its use as a solar energy concentrating collector began in various centres following a more recent paper by Winston in 1974. The basic compound parabolic collector consists of parabolic reflectors which funnel the radiation from aperture to absorber. The right and left half belong to different, but similar parabolas with the axis of each inclined at the same angle (the collector acceptance half angle) with the collector axis. The focus of the right hand parabola is at the intersection of the left hand parabola with the base line along which the absorber plate is placed. No long wave radiation can leave the collector outside its acceptance angle, so that the overall performance of a conventional collector with a good selective surface is obtained without any costly chemical coating processes.
To achieve even moderate concentration ratios the depth of the collector, H, must be several times greater than the aperture, and this resulted in the development of the truncated CPC. The top section of the reflectors in FIG. 1. do not intercept much radiation and can be omitted without any significant loss in concentration. Nevertheless, even a truncated CPC would be over 300mm deep for 3:1 concentration ratio.
Another type of concentrating collector is based on the use of a logarithmic spiral, rather than a parabola, and collectors based on this principle are available in the United States. A U.K. design was awarded a prize in the 1975, Copper Development Association competition. The direct beam radiation entering the collector is reflected to the absorber, which normally consists of a bled pipe. Both the compound parabolic collector and the spiral collector have a very limited potential in the U.K. because of their relatively large depth compared with conventional flat-plate collectors and the fact that they cannot make effective use of diffuse radiation.
An object of the present invention is to provide a solar heater which substantially overcomes the drawbacks of these known collectors, and is relatively simple in construction leading to considerable economies in mass production.